This invention pertains to the field of antennas and, more particularly, to a dual mode meander line loaded antenna (MLA) providing simultaneous dual wideband and narrowband operation.
Existing MLA antennas are typically narrow band antennas. For many narrowband military and commercial applications, radio frequency signals can appear unexpectedly across a wide frequency range. These existing MLA antennas are not capable of working effectively in such an environment.
In the prior art, efficient antennas have typically required structures with minimum dimensions on the order of a quarter wavelength of their intended radiating frequency. These dimensions allowed the antennas to be easily excited and to be operated at or near their resonance, limiting the energy dissipated in resistive losses and maximizing the transmitted energy. These antennas tended to be large in size at their resonant wavelengths. Further, as the operating frequency decreased, the antenna""s dimensions were increased proportionally. In order to address the shortcomings of traditional antenna design and functionality, the meander line loaded antenna (MLA) was developed. The basic theory and design of the meander line loaded antenna is presented in U.S. Pat. No. 5,790,080.
An example of a basic prior art MLA, also termed a varied impedance transmission line antenna, is shown in FIG. 1. The antenna 100 consists of two vertical sections (i.e., plates) 102 and a horizontal section 104. The vertical and horizontal sections 102, 104, respectively, are separated by gaps 106. Also part of the antenna 100 are the meander lines 200 (FIG. 2), which are typically connected between the vertical and horizontal sections 102, 104 at the gaps 106.
The meander line 200 is designed to adjust the electrical (i.e., resonant) length of the antenna 100. The design of the meander slow wave structure 200 is such that it is possible to switch lengths of the meander line 200 in or out of the circuit quickly and with negligible loss, in order to change the effective electrical length of the antenna 100. This switching is possible because the active switching devices (not shown) are always located in the high impedance sections of the meander line 200. This keeps the current through the switching devices (not shown) low and results in very low dissipation losses in the switches, thereby maintaining high antenna efficiency. Switching of sections of a meander line using mechanical, electrical, microelectromechanical systems (MEMS) switches, or the like, are well known to those skilled in the antenna design arts.
The basic antenna of FIG. 1 can be operated in a loop mode that provides a xe2x80x9cfigure eightxe2x80x9d coverage (i.e., radiation) pattern. Horizontal polarization, loop mode, is obtained when the antenna is operated at a frequency such that the electrical length of the entire line including the meander lines 200 is a multiple of full wavelength as shown in FIG. 3C.
The antenna can also be operated in a vertically polarized mode, monopole mode, by adjusting the electrical length to an odd multiple of a half wavelength at the operating frequency, FIGS. 3B and 3D, respectively. The meander lines 200 can be tuned using electrical or mechanical switches (not shown) to change the mode of operation at a given frequency or to switch frequencies using a given mode.
The meander line loaded antenna allowed the physical antenna dimensions to be significantly reduced in size while maintaining electrical lengths that were still multiples of a quarter wavelength. Antennas and radiating structures built using this design approach operate in the region where the limitation on their fundamental performance is governed by the Chu-Harrington relation:
Efficiency=FV2Q 
where: Q=Quality Factor;
V2=Volume of the structure in cubic wavelengths; and
F=Geometric Form Factor (F=64 for a cube or a sphere)
Meander line loaded antennas achieve the efficiency limit of the Chu-Harrington relation while allowing the antenna size to be much smaller than a wavelength at the frequency of operation. Height reductions of 10 to 1 over quarter wave monopole antennas can be realized, while achieving comparable gain.
But, the existing MLA antennas are narrowband antennas. For many narrowband military and commercial applications where signals can appear unexpectedly across a wide frequency range, the existing MLA antennas are not desirable.
U.S. Pat. No. 5,790,080 entitled MEANDER LINE LOADED ANTENNA, describes an antenna that includes one or more conductive elements for acting as radiating antenna elements, and a slow wave meander line adapted to couple electrical signals between the conductive elements. The meander line has an effective electrical length that affects the electrical length and operating characteristics of the antenna. The electrical length and operating mode of the antenna may be readily controlled.
U.S. Pat. No. 6,034,637 entitled DOUBLE RESONANT WIDEBAND PATCH ANTENNA AND METHOD OF FORMING SAME, describes a double resonant wideband patch antenna that includes a planar resonator forming a substantially trapezoidal shape having a non-parallel edge for providing a substantially wide bandwidth. A feed line (107) extends parallel to the non-parallel edge for coupling while a ground plane extends beneath the planar resonator for increasing radiation efficiency.
U.S. Pat. No. 6,008,762 entitled FOLDED QUARTER-WAVE PATCH ANTENNA, describes a folded quarter-wave patch antenna which includes a conductor plate having first and second spaced apart arms. A ground plane is separated from the conductor plate by a dielectric substrate that is approximately parallel to the conductor plate. The ground plane is electrically connected to the first arm at one end and a signal unit is electrically coupled to the first arm. The signal unit transmits and/or receives signals having a selected frequency band. The folded quarter-wave patch antenna can also act as a dual frequency band antenna. In dual frequency band operation, the signal unit provides the antenna with a first signal of a first frequency band and a second signal of a second frequency band.
Each antenna of the prior art devices requires the use of multiple, separate wideband and narrowband antennas. What is needed is a means to provide a wideband receive capability, while simultaneously receiving narrowband signals on the same MLA antenna. Such an antenna should be simple and inexpensive to manufacture and also enable retrofitting of existing MLA antennas.
In accordance with the present invention, there is provided a dual mode, meander line loaded antenna (MLA) having an additional wideband plate or hat located above the horizontal top surface of the antenna. The upper plate is spaced a predetermined distance above the MLA and held in place by dielectric blocks of a predetermined thickness. By properly spacing the additional plate, simultaneous wideband and narrowband reception can occur. The added upper plate generally does not interfere with the usual narrowband operation/reception of the original antenna structure. The modified antenna can accept radio frequency signals appearing unexpectedly across a wide range of frequencies. The additional upper plate can be retrofitted to existing meander line loaded antennas to modify them for dual mode operation. The narrowband/wideband dual mode antenna operates simultaneously a wideband signal and a narrowband signal.
It is therefore an object of the invention to provide a MLA antenna capable of simultaneous dual mode operation. One of the facets of the invention is to insert a structure that does not effect the existing tunable high frequency MLA antenna usage. In one embodiment, the additional structure is placed a few inches above the horizontal MLA section. Using the voltage induced between the structure and the horizontal section as the input to a high impedance field effect transistor (FET), the incidence vertical electric fields are detected simultaneous with the normal narrowband operation of the MLA antenna.
It is a further object of the invention to provide a MLA antenna where the simultaneous dual operating modes are a broadband and a narrowband mode of operation. It is another object of the invention to provide a MLA antenna suitable for use in environments where signals may appear unexpectedly over a wide range of frequencies. It is a still further object of the invention to provide a MLA antenna suitable for use in wideband signal acquisition applications, while simultaneously performing direction finding.
Another object is a narrowband/wideband dual mode antenna comprising a meander line loaded antenna (MLA) having a pair of vertical sections disposed substantially perpendicular to a ground plane, one of the pair of vertical sections being electrically connected to the ground plane. There is a substantially horizontal top section disposed above and substantially perpendicular to the pair of vertical sections, each end of the top section being proximate one of the pair of vertical sections and separated therefrom by a gap. One or more meander line elements are proximate at least one of the gaps and operatively connected to one of the vertical sections and to the top section. A wideband plate is disposed a predetermined distance above and electrically isolated from the horizontal top section. And, there is a feed means for accepting a voltage induced between the wideband plate and the top section by an incoming signal.
And another object is the narrowband/wideband dual mode antenna, wherein the feed means is a high impedance amplifier. Furthermore, wherein the high impedance amplifier is a field effect transistor (FET) having a gate, a drain, and a source, wherein the gate is connected to the wideband plate, the source is connected to the top section, and the drain is connected to the vertical section electrically connected to the ground plane.
Yet a further object is the narrowband/wideband dual mode antenna wherein the electrical isolation between the wideband plate and the horizontal top section is provided by a dielectric material. In one embodiment there is at least one dielectric block, although other separating means are within the scope of the invention. The dielectric material can be any high-frequency dielectric material such as Teflon7, polyethylene, and phenolic.
An additional object is the narrowband/wideband dual mode antenna wherein the meander line loaded antenna is a tunable, varied impedance transmission line. And, wherein the tunable, varied impedance transmission line comprises switching means for selectively connecting and disconnecting at least a portion of the transmission line.
Another object is the narrowband/wideband dual mode antenna wherein the meander line is a printed circuit structure.
And, an object includes the narrowband/wideband dual mode antenna wherein the meander line elements are electrically isolated from the horizontal top section by a dielectric material. And, the narrowband/wideband dual mode antenna further comprising at least one dielectric bar disposed between at least two of the structures, the ground plane, at least one of the pair of vertical sections, and the substantially horizontal top section. Further comprising fastening means for securing at least one dielectric bar to one of the structures, wherein the fastening means comprises at least one from the group of screw, bolt, and adhesive.
An object of the invention is a method for operating dual bandwidths using a meander line loaded antenna (MLA), comprising the steps of providing an MLA having a pair of vertical sections disposed substantially perpendicular to a ground plane, one of the pair of vertical sections being electrically connected to the ground plane, with a substantially horizontal top section disposed above and substantially perpendicular to the pair of vertical sections, each end of the top section being proximate one of the pair of vertical sections and separated therefrom by a gap, and with one or more meander lines proximate at least one of the gaps and operatively connected to at least one of the vertical sections and to the top section. Disposing a wideband plate at a predetermined distance above and electrically isolated from the horizontal top section by at least one dielectric block, and securing the wideband hat to the dielectric block. Providing wideband feed means electrically connected to the horizontal top section and the wideband hat for accepting a voltage induced between the wideband hat and the horizontal top section by an incoming radio frequency signal, whereby the dual mode antenna receives simultaneous broadband and narrowband signals.
And a further object is the method for operating dual bandwidths, further comprising the step of electrically connecting the vertical section connected to the feed means, wherein the connecting does not cross the gap.
It is an additional object of the invention to provide a MLA antenna incorporating a wideband mode plate to allow simultaneous dual mode operation. It is another object of the invention to provide a MLA antenna having a wideband hat section that may be retrofitted to existing narrowband meander line loaded antennas.